Method and device for evaluating a liquid dosing process

ABSTRACT

The invention relates to a method for evaluating a liquid dosing process in a container which is at least partially filled with a gas. According to the inventive method, a temporal course of at least one state variable p of a medium contained in said container is determined essentially over the entire duration of the dosing process. The temporal course ( 40; 40′ ) of the at least one state variable (p) is graphically or mathematically compared with a pre-determined state variable nominal range ( 42; 42′; 242 ) by means of a correlation method, and an evaluation result (S 6 , S 14 , S 16 ) is obtained according to the results of the comparison.

DESCRIPTION

[0001] The present invention relates to a method and a device forevaluating a liquid dosing process in a container which is at leastpartially filled with a gas.

[0002] Liquid dosing processes are often part of mixing or analyzingprocedures in which exact doses of liquids are taken from liquidquantities and mixed together, for example. Liquid dosing processes areroutine in chemical, pharmaceutical, medical and human biologicalprocesses. Many of these dosing processes are part of a manufacturingprocess for production of pharmaceutical or medical active ingredientsand drugs or they make a contribution toward making a medical diagnosisof diseases. Undetected faulty liquid dosing can therefore result inproducts which are objectionable or even harmful for the health ofliving creatures, in particular humans. However, even if faulty liquiddosing is detected in an operational or clinical quality assurancestage, there is still the risk of wasting valuable substances, whichunder some circumstances are available only in limited amounts due to anunnecessarily large number of reject doses.

[0003] Therefore, it is extremely important to be able to evaluateliquid dosing processes as soon as possible with the greatest possiblecertainty as to whether the process is fault-free.

[0004] Various methods are known from the state of the art forevaluating a liquid doing process, e.g., for an aspiration process,i.e., intake of a liquid and for a dispensing process, i.e., dispensinga liquid in pipetting.

[0005] In an aspiration process, the tip of the pipette is firstimmersed in the liquid to be taken up. Therefore, a quantity of gaspresent in a liquid holding space bordered by the pipette tip opening,the pipette tip inside wall and the plunger is sealed off and separatedfrom the gas volume of the environment so that the quantity of gaspresent in the pipette tip remains approximately constant, i.e., apartfrom evaporation and condensation processes. Due to an intake movementof the pipetting plunger away from the pipette tip, the volume of thequantity of gas that is sealed off is increased, so that the pressure ofthe gas in the liquid holding space drops. Beyond a certain pressuredifference between the gas pressure in the liquid holding space and theambient gas pressure, liquid begins to flow through the pipette tipopening into the liquid holding space. Due to the liquid flowing in, therate of change in gas volume declines and thus the rate of change in gaspressure in the liquid holding space also drops.

[0006] In the known methods of evaluating a liquid dosing process,monitoring is provided to ascertain whether the gas pressure in theliquid holding space drops below a predetermined limit value. In someprocesses, the rate of change in the pressure of the gas enclosed in theliquid holding space is also observed in addition to whether theprevailing value drops below a limit value, i.e., a check is performedto determine whether the gas pressure in the liquid holding spacechanges by a predetermined amount in a predetermined period of time.This check can be performed graphically by comparing the slope of apressure-time curve with a predetermined slope or analytically bycomparing corresponding pressure-time value pairs.

[0007] For the dispensing process in which the volume of a quantity ofgas enclosed between a liquid that has been taken up and the pipettingplunger is to be reduced by a displacement movement of the pipettingplunger toward the pipette opening, the evaluation process describedpreviously is also applicable accordingly. It is true in general thatthe dosing process is evaluated as fault-free if the pressure of the gasenclosed in the liquid holding space has reached, exceeded or fallenbelow a certain limit value and/or if the change in pressure over timereaches, exceeds or falls below a certain limit value.

[0008] One disadvantage of this state-of-the-art method is that theevaluation as to whether or not the liquid dosing process has takenplace without any errors is based on only a few measured values, whichare usually measured at the beginning of the dosing process. An errorwhich occurs after reaching the gas pressure limit value is no longerdetected by this method. Such an error may occur, for example, when thepipette tip opening is clogged due to a solid present in the liquidduring the inflow of liquid into the pipette tip. This may be the casewhen dosing blood, for example, if coagulated components are present inthe liquid blood.

[0009] Therefore, the object of the present invention is to provide ateaching which will make it possible for those skilled in the art toreliably evaluate liquid dosing processes with regard to theirsuccessful completion and to promptly detect faulty dosing.

[0010] According to a first aspect of the present invention, this objectis achieved by a method of evaluating a liquid dosing process in acontainer which is filled at least partially with a gas, preferably withair, in particular an aspiration and/or dispensing process in pipetting,in which process a time characteristic of at least one state variable ofa medium that is present in the container is detected essentially overthe entire duration of the dosing process, in which moreover essentiallythe entire chronological course of the at least one state variable iscompared with a predetermined state variable setpoint range, and inwhich an evaluation result is output dependent on the result of thecomparison.

[0011] Although in the acknowledgment of the state of the art, a processfor evaluating a pipetting procedure has been described, the inventivemethod is not limited to pipettes as containers, but instead may beapplied to any containers. Due to the fact that at least one statevariable of a medium present in the container is detected overessentially the entire duration of the dosing process, information isavailable regarding the filling level of the container essentially forthe entire dosing process and can be used to evaluate it. By comparingessentially the entire chronological course of the at least one statevariable with a state variable setpoint range, abnormal values of the atleast one state variable occurring at any point in time during thedosing can be detected, and thus the dosing process can be evaluatedreliably.

[0012] The state variable setpoint range may be, for example, anidealized state variable characteristic which is provided with anadditional tolerance allowance under some circumstances.

[0013] The state variable can be detected in any medium present in thecontainer. For example, the state variable may be the hydrostaticpressure of the liquid dosed into an open beaker or a bottle, thispressure being measured at the lowest point in the beaker or bottle.Frequently, however, containers having a gas space sealed off during thedosing process are used for liquid dosing, as is the case, for example,in pipetting with pipette tips. In such containers, by detecting atleast one state variable of the gas present in the container, it ispossible to obtain a particularly accurate result because in contrastwith the liquid flowing in or out, the quantity of gas enclosed in thecontainer is influenced almost exclusively by the liquid to be dosed,and any influence by the environment of the container is virtually ruledout.

[0014] Another advantage of detecting at least one state variable of thegas present in the container is that in this way it is also possible toevaluate dosing processes using smaller dosing quantities than in thecase of determination of a state variable of the dosed liquid itself,because the liquid is subject to adhesion interactions and/or frictionalinteractions with the wall of the container to an even greater extentthan is the gas. These interactions become negligible only beyond acertain minimum quantity of liquid.

[0015] The inventive method can be implemented with any type of gas,i.e., in any type of gas atmosphere. In the simplest and most commoncase, the dosing process is carried out in ambient air, which is why thecontainers in this case are filled with air. However, it is alsoconceivable for liquids whose contact with air or oxygen is notdesirable to be dosed. In this case, the inventive method may also beused in dosing in an inert or quasi-inert atmosphere such as an argon,nitrogen or carbon dioxide atmosphere.

[0016] As described above, the hydrostatic pressure may be used as thestate variable for a measurement in a liquid present in the containerand the gas pressure and/or temperature may be used for a measurement inthe gas. Since the quantity of gas present in the container, i.e., thegas mass, remains approximately constant during the dosing process inmany dosing containers, but the volume of the quantity of gas is altereddue to movement of a plunger, therefore the pressure changes with thevolume and, depending on the execution of the dosing process, thetemperature of the gas also changes. In the case of especially slowchanges in gas volume in the container, it is possible to approximatelyassume an isothermal change in volume. In this case, only measurement ofthe pressure has any relevance. When there are particularly rapidchanges in volume, it is possible in first approximation to assume anadiabatic change in state, which is why with a knowledge of theadiabatic exponent assigned to the gas, either the pressure or thetemperature can be determined as additional state variables. Thegreatest accuracy and reliability, however, are obtained by detectingboth the pressure and temperature of the gas because then there can bemutual monitoring of the functional reliability of the state variabledetection sensors.

[0017] For determination of a state variable, it is sufficient todetermine a variable which changes in a known relationship to the statevariable.

[0018] The state variable setpoint range is advantageously defined atleast for the entire duration of the liquid dosing process. In thiscase, it is possible to evaluate the liquid dosing process not only incertain intervals of time but in fact at any point in time during thedosing process.

[0019] However, this does not mean that the state variable setpointrange is defined only for the duration of the change in the quantity ofliquid in the container. It may also be appropriate to also determinethe state variable before and/or after the phase of the change in thequantity of liquid in the container and also to extend the statevariable setpoint range to these intervals of time accordingly. Thus, atransport phase, if any, between the aspiration phase and thedispensation phase can be monitored, e.g., for loss of liquid due toformation of droplets and loss of droplets or even loss of the pipettetip.

[0020] The precise procedure for determining such processes before andafter the liquid dosing is described further below on the basis of oneexemplary embodiment.

[0021] According to a first preferred embodiment of the presentinvention, the state variable setpoint range may be defined as followinga setpoint curve, in which case then to evaluate the dosing process itis determined whether the time characteristic of the at least one statevariable is within the state variable setpoint range and an evaluationresult is output dependent on of the result of the determination. Thisis a comparison which is very simple to perform and with which a dosingprocess can be evaluated reliably.

[0022] For reasons of the greatest possible clarify and ease ofunderstanding of the evaluation results obtained, the state variablesetpoint range may advantageously be defined in such a way that theliquid dosing process is evaluated as error-free as long as the timecharacteristic detected for the at least one state variable is withinthe state variable range, and it is evaluated as faulty if it is foundthat the time characteristic determined for the at least one statevariable is outside of the state variable setpoint range in at leastsome segments.

[0023] For example, a pipette opening may be clogged temporarily by asolid or its cross section may be reduced, and after a period of delay,the solid is rinsed away by the liquid flowing in or out. In this case,the gas pressure in the interior of the pipette tip would drop sharply,e.g., in an aspiration process (and/or the gas temperature would dropsharply), so that the state variable would leave its setpoint range.After eliminating the problem, the state variable may again assumevalues within the setpoint range. However, since undefined flowconditions prevailed at the pipette tip while the problem was occurring,it is advisable to evaluate a pipetting process as faulty if it is foundthat the time characteristic determined for the at least one statevariable is outside of the state variable setpoint range in at leastsome segments.

[0024] Another advantage of the inventive method is the possibility ofdiagnosing an error if it occurs to determine the type of error inaddition to evaluating a correct sequence of the dosing process. To doso it is advantageous that when the time characteristic of the at leastone state variable is determined as being outside of the state variablesetpoint range in at least some segments, it is determined whether in atleast some segments the characteristic of the at least one statevariable is in at least one error range of a plurality of error rangesof a state variable value range which is outside of the state variablesetpoint range. Then an error message is output dependent on at leastone error range that has been passed through.

[0025] If the time characteristic of the at least one state variableleaves the state variable setpoint range, then the time characteristicof the at least one state variable is in a state variable value rangewhich is outside of the state variable setpoint range. Then differenttypes of errors usually occur at different times and/or lead todifferent deviations in the state variable value from the state variablesetpoint range. It is therefore possible to subdivide the state variablevalue range surrounding the state variable setpoint range into at leastone error range, preferably a plurality of error ranges. Each errorrange is advantageously assigned precisely one error, or under somecircumstances it may also be assigned a plurality of errors. In the caseof a plurality of error ranges, they are differentiated from one anotherin time and/or through state variable threshold values, optionallyvariable over time.

[0026] Likewise, the state variable setpoint range may be delimited byan upper and a lower threshold curve from the remaining state variablevalue range. The upper threshold curve is the threshold curve whichlimits the state variable setpoint range toward higher state variablevalues. The lower threshold curve is accordingly the threshold curvewhich limits the state variable setpoint range toward lower statevariable values. The threshold curves may be functions of time and infact they usually are because the state variable setpoint range usuallyfollows a nontrivial setpoint curve. In this case, the determination ofwhether the time characteristic of the at least one state variable iswithin the predetermined state variable setpoint range can be performedeasily by a comparison of the time characteristic with the upperthreshold curve and the lower threshold curve.

[0027] As an alternative to this, the determination of whether the timecharacteristic of the at least one state variable is within thepredetermined state variable setpoint range may also be performed byimage processing. An image processing determination method isfacilitated by the inventive method inasmuch as the databases used inthe process, e.g., the time characteristic of at least one statevariable, state variable setpoint range and, if desired, a plurality oferror ranges are especially suitable for a graphic representation andanalysis.

[0028] The quality of the evaluation of the liquid dosing processachieved with the inventive method depends to a great extent on thestate variable setpoint range used for the evaluation. If the statevariable setpoint range has been chosen to be very broad, there is therisk that dosing processes that are already faulty might be evaluated asfault-free. Conversely, if the state variable setpoint range is chosento be very narrow, this entails the risk that fault-free dosingprocesses might be evaluated as faulty.

[0029] A state variable setpoint range of a certain liquid dosingprocess which is particularly suitable for the evaluation of liquiddosing processes can be obtained by repeatedly performing essentiallythe same liquid dosing process using essentially the same processparameter and thereby detecting the time characteristic of the at leastone state variable. The phase “essentially the same process parameter”means that, if possible, the same liquid (or at least a liquid havingessentially the same viscosity, surface tension, etc.) is dosed atessentially the same ambient temperature into essentially the samecontainer, i.e., a container of the same design, e.g., the same ordernumber from the same manufacturer, in essentially the same gasatmosphere with essentially the same operating settings of a dosingdevice. The operating settings of a dosing device include, for example,the dosing rate in volume of liquid per unit of time or weight of liquidper unit of time.

[0030] The scattering which occurs in practical use of a dosing deviceor the process parameters to be attributed to an exemplary scattering ofthe dosing device, e.g., measurement temperature, dosing rate and, asmentioned above, the shape of the container, shall thus be subsumedunder “essentially the same” so that the setpoint range thus determinedtakes the scattering in parameters into account.

[0031] Assuming each individual dosing process has proceededfaultlessly, one obtains after repeatedly performing the liquid dosingprocess a set of time characteristics of the at least one state variablewhose envelope curve can be used as the basis for additionalperformances of this liquid dosing process as the state variablesetpoint range. Depending on the safety relevance of the quantity ofliquid dosing or depending on the value of the liquid dosing, theenvelope curve of most of the time characteristics of the at least onestate variable may be increased or decreased by a tolerance amount, andthe envelope curve thus increased or reduced may be used as the setpointrange.

[0032] As an alternative to that, the set of time characteristics of theat least one state variable may also be combined into a reference curve,e.g., by forming an average. This predetermined reference curve,provided with a bilateral tolerance field (±n−6), may also serve as astate variable setpoint range.

[0033] According to another preferred embodiment of the inventivemethod, a degree of correspondence of the time characteristic of the atleast one state variable with the predetermined reference curve may bedetermined from the time characteristic of the at least one statevariable by correlation calculation methods and an evaluation resultpertaining to the dosing process may be output as a function of theresult of the determination. By using correlation calculation methods,very precise comparisons of the time characteristic of the at least onestate variable with the predetermined reference curve are possible. Inaddition, by performing a correlation calculation process and by storinga reference curve for certain operating parameters, it is possible toreduce the memory space required for storage of the state variablesetpoint range and the computation time required for the comparison witha prevailing state variable characteristic may also be reduced.Therefore, the dosing process to be evaluated ca n also take place morerapidly. The calculated correlation coefficient may be used as a qualitycharacteristic.

[0034] Known methods, e.g., fast Fourier transform, polynomialregression, regression methods in general, wavelets and differentiation,may be used as the correlation methods.

[0035] Such correlation calculation methods usually deliver the degreeof correspondence between two curves or point curves as a numericalvalue. The dosing process to be investigated may then be evaluated asfaulty, e.g., if the degree of correspondence determined is outside apredetermined degree of correspondence setpoint range. The evaluationresult can be obtained particularly rapidly by this comparison of anumerical value with a predetermined value range, which is of greatimportance in view of the short time available in industrial dosingprocesses.

[0036] In addition, an error which occurs when the degree ofcorrespondence is detected as being outside the predetermined degree ofcorrespondence setpoint range can be investigated in greater detail by amore extensive diagnostic comparison method. In particular thisdetermines whether the degree of correspondence is in an error range ofa plurality of error ranges of a degree of correspondence value rangewhich is outside the degree of correspondence setpoint range. Then anerror message is output as a function of the error range in which thedegree of correspondence is located. It is therefore possible to rapidlyand reliably detect a systematic error in the dosing system andeliminate it. To do so, error ranges within the total degree ofcorrespondence value range can be determined, e.g., in experiments andcertain errors and/or error groups may be assigned to them, so thatunder some circumstances it is possible to make a statement regardinghow critical the particular error is.

[0037] To be able to save on further computation time and further memoryspace, it is sufficient if the correlation calculation method uses asthe input variable interpolation points from the time characteristic ofthe at least one variable state and from the reference curve. If thereis a sufficiently small distance between the interpolation points, thecomputation time and the required memory space can be reducedconsiderably without any sacrifice of accuracy in the evaluation result.

[0038] According to another aspect of the invention, the objectdescribed above is also achieved by a device for evaluating a liquiddosing process in a container filled at least partially with gas,preferably air, using the method described above whereby the devicecomprises at least one sensor for detecting the time characteristic ofthe at least one state variable, a data memory for storing apredetermined state variable setpoint range of state variable valuesdetected by the sensor and, if desired, points in time of detection ofthe individual state variable values, a data processing unit forcomparison of the time characteristic of the least one state variablewith the predetermined state variable setpoint range and an output unitfor output of an evaluation result as a function of the result of thecomparison by the data processing unit.

[0039] The at least one sensor is used to detect the time characteristicof the at least one state variable. This determination may be performedcontinuously or in individual measurements performed in time intervals,whereby the interval between two individual measurements is small incomparison with the total duration of the liquid dosing process.

[0040] The predetermined state variable setpoint range is stored in thedata memory. In addition, the state variable values detected by thesensor are also stored in the data memory.

[0041] For example, a time characteristic may be formed from a pluralityof individual measurements by assigning to each measurement a machinestate or a container state which is characteristic of the dosingprocess, e.g., the position of a movable plunger relative to theremaining container. The position of the plunger is equivalent to apoint in time at least during the phase in which the plunger is moving.

[0042] The device may also include a clock. If desired, as analternative or in addition to the machine states mentioned above, thepoints in time assigned to a state variable determination may also bestored themselves. Storage of state variables together with thedetermination points in time assigned to them or machine statesequivalent to them is necessary, e.g., when the determination of statevariable values by at least one sensor is not performed at constantintervals. If state variable values are determined at constant intervalsin time, however, then the storage of determination times may be omittedbecause the determination time can be determined from the sequenceposition of a state variable value in a series of state variable values.

[0043] In addition, this device also includes a data processing unitwhich uses data stored in the data memory for comparison of the timecharacteristic of the at least one state variable with the statevariable setpoint range.

[0044] Finally, an output unit is used for output of an evaluationresult which is obtained as a function of the result of the comparisonby the data processing unit. The output unit may use alphanumericcharacters and/or graphic elements, e.g., colored and/or structuredlines and/or areas for output of the evaluation result and if desiredfor representing the time characteristic of the state variable as wellas the state variable setpoint range.

[0045] In addition to the state variable setpoint range, a plurality ofpredetermined error ranges may also be stored in the data memory, eacherror range being assigned at least one possible error of the dosingprocess. In this way the data processing unit can diagnose the error(s)of the dosing process in question.

[0046] In addition, the device for creating the state variable setpointrange may include an editing unit with which a state variable setpointrange can be created, e.g., on the basis of a set of timecharacteristics.

[0047] The editing unit may include an input unit connected to it. Viathis input unit it is possible to enter, for example, numerical valueswhich define tolerance ranges by which an envelope curve of the set oftime characteristics is widened or narrowed with respect to the set ofcurves.

[0048] As an alternative or in addition to that, the output unit of thedevice may be a graphic output unit, in which case then the input unitcan also graphically determine a state variable setpoint range. Thisgraphic method in which the set of time characteristics of the statevariable and a state variable setpoint range, for example, arerepresented visually together is a particularly simple but neverthelessvery effective option for creating a state variable setpoint range.However, with the editing unit a reference curve can be createdgraphically from the set of time characteristics. However, this may alsobe accomplished with greater accuracy by a computation process.

[0049] According to the preferred embodiments of the inventive methodpresented previously, the data processing unit may determine whether thetime characteristic of the at least one state variable is within thepredetermined state variable setpoint range. As an alternative or inaddition to that, the data processing unit may also be designed toperform a correlation calculation process for determining a degree ofcorrespondence of the time characteristic of the at least one statevariable with a predetermined reference curve as the state variablesetpoint range. It is advantageous here if a predetermined degree ofcorrespondence setpoint range is stored in the data memory so that thedegree of correspondence thus determined can be compared with it. Inthis way it is possible to determine whether the degree ofcorrespondence thus determined is within the predetermined degree ofcorrespondence setpoint range.

[0050] The above mentioned error ranges which may be stored in the datamemory for error diagnosis may be, for example, degree of correspondencevalue ranges. Then a certain error and/or a certain error group isassigned to a certain range of degree of correspondence values.

[0051] It should be pointed out explicitly here that the two preferredembodiments mentioned above for increasing the evaluation reliabilitymay also be used in combination for one and the same dosing process.

[0052] As already stated previously, the inventive method may be used toevaluate a liquid dosing process with any containers, any liquids and inany gas atmosphere. The same thing is also true of the device describedabove. However, this method is particularly suitable for pipettingprocesses, which is why the inventive device is preferably used on apipetting system and/or the inventive method preferably evaluates apipetting process on a pipetting system.

[0053] It is conceivable here that, in the case when a dosing that isevaluated as faulty, the method and/or the device may also order and/orperform measures in addition to merely performing an evaluation. Thisincludes, for example, stopping a certain dosing process[,] changingcertain pipette tips, discarding a dosing process, e.g., aspiration andrepeating this dosing process.

[0054] The present invention is explained in greater detail below on thebasis of the accompanying drawings, which show:

[0055]FIGS. 1a-1 e phases of an aspiration process in pipetting,

[0056]FIG. 2 a method of evaluating a liquid dosing process according tothe state of the art,

[0057]FIG. 3 a graphic representation of a time characteristic of thepressure of a gas present in the liquid holding space of a pipette tipin an aspiration and dispensing process, a state variable setpoint rangeaccording to the first preferred embodiment of this invention as well aserror ranges surrounding the setpoint range,

[0058]FIG. 4 a flow chart describing the first preferred embodiment ofthe inventive method,

[0059]FIG. 5 a diagram showing the creation of state variable setpointranges of the preferred embodiments of the present invention.

[0060] On the basis of FIGS. 1a through 1 e, a liquid aspiration processin pipetting on which the exemplary embodiment of the present inventionis based will be explained briefly on the basis of schematic diagrams.

[0061]FIG. 1a shows a schematic cross section through a pipette tip 10which moves toward the liquid level 14 a of a liquid 14 in the directionof the arrow 12. A pressure detecting sensor 22 which detects thepressure of the gas present in the liquid holding space 20 is situatedin the liquid holding space.

[0062] In FIG. 1b the opening 10 a in the pipette tip 10 has reached theliquid level 14 a. Therefore, the quantity of gas present in the liquidholding space 20 of the pipette tip 10 is separated from the ambient airand remains essentially constant apart from evaporation and condensationprocesses. The pipette tip 10 is also lowered in the direction of thearrow 12.

[0063] In FIG. 1c the pipette tip 10 has reached its lowest point and itremains with the opening 10 a immersed in the liquid 14. The plunger 16is now moved in the direction of the arrow 18, but no liquid has yetflowed into the liquid holding space 20 because of friction and surfacetension effects.

[0064] It can be seen in FIG. 1d that the uptake of liquid 14 by thepipette tip 10 through the opening 10 a has already begun. Then theplunger 16 is not moved any further (FIG. 1d′), so the volume of theliquid holding space 20 of the pipette tip 10 is not increased further.However, because of the negative pressure prevailing in the liquidholding space 20 with respect to the environment, liquid 14 continues toflow into the liquid holding space 20 until an equilibrium isestablished.

[0065] The aspiration process is concluded in FIG. 1e. The pipette tip10 has been lifted out of the liquid 14. A certain volume of liquid 14is in the liquid holding space 20 of the pipette tip 10 and is heldthere due to the negative pressure of the gas enclosed between theplunger 16 and the liquid volume with respect to the environment. Inaddition, friction and adhesion effects between the liquid volume andthe wall of the pipette tip 10 also contribute to the liquid volumeremaining in the pipette tip 10.

[0066]FIG. 2 shows a pressure-time diagram 30 of a state-of-the-artprocess for evaluating a liquid dosing process. The time t is plotted onthe abscissa of the diagram in FIG. 2 and the pressure p of the quantityof gas present in the liquid holding space 20 is plotted on theordinate.

[0067] The liquid dosing process is evaluated in such a way as todetermine whether the pressure-time curve achieves a slope a in at leastsome segments, i.e., whether the rates of change in gas pressure reachesa predetermined value which is proportional to tan a in at least onesegment and/or whether the pressure in the liquid holding space of thepipette tip falls below a predetermined limit value p* in aspiration. Ifthe slope α is reached by the pressure-time curve in a period of timeclose to the start of aspiration and/or if it falls below the limitvalue p*, the aspiration process is evaluated as faulty. If one of theaforementioned conditions is not met, the aspiration process isevaluated as faulty.

[0068] In FIG. 3 the gas pressure in the liquid holding space 20 of thepipette tip 10 which is detected with the pressure detecting sensor 22of FIGS. 1a through 1 e during an aspiration process is represented bythe dotted line 40 in time range A. This curve of pressure over time isplotted in a coordinate system. The time t is plotted on the abscissaand the pressure p of the gas in the liquid holding space 20 is plottedon the ordinate.

[0069] In addition, a pressure setpoint range 42 which follows asetpoint curve is also plotted in this coordinate system. In thisdiagram, the error ranges 44, 46 and 48, which are below the pressuresetpoint range 42, as well as the error ranges 50, 52, 54, 56 and 58,which are above the pressure setpoint range 42, i.e., toward higherpressures, are outside the pressure setpoint range 42.

[0070] In FIG. 3, the time range A, the curve 40 of the gas pressureover time in the entire definition range of the pressure setpoint range42 is within this range, which is why the dosing process in question isevaluated as faulty.

[0071] For a better understanding of the curve 40 of the gas pressure inthe liquid holding space 20 of the pipette tip 10 of FIGS. 1a through 1e over time, the following brief explanation is given.

[0072] The curve begins at time t=0 at ambient pressure ρ₀. In the firstsegment 40 a the pressure remains constant. This corresponds to thestate shown in FIG. 1a where the volume of the liquid holding space 20remains constant. As soon as the opening 10 a in the pipette tip 10 asshown in FIG. 1b has reached the liquid level 14 a, there is at first aslight reduction in pressure because of adhesion in the contact with theliquid surface which is then superimposed on the growing static pressurein the pipette tip as it is immersed to an increasing extent. Botheffects are comparatively minor and therefore have not been entered intoFIG. 3.

[0073] At a point in time which corresponds to FIG. 1c, the plunger 16is moved upward in the direction of the arrow 18 at a constant rate,with the result that the pressure drops drastically. This phase of thedrastic drop in pressure represented by segment 40 c ends at point 40 d,at which the liquid begins to flow into the liquid holding space 20 ofthe pipette tip. In the area 40 e adjacent to the point 40 d, anyfurther increase in the gas volume of the quantity of gas enclosed inthe liquid holding space 20 induced by the movement of the plunger 16 isdiminished due to the liquid flowing into the liquid holding space 20,i.e., a liquid interface follows the plunger as it is raised. A dynamicequilibrium is established approximately between the increase in gasvolume caused by the plunger and the reduction in volume caused by theinflowing liquid.

[0074] The movement of the plunger and thus the increase in volume ofthe liquid holding space end at time t₁ at point 40 f (FIG. 1d). Thenegative pressure of the gas which is still present in the liquidholding space 20 with respect to the ambient gas of the pipette tip 10allows more liquid to flow in the liquid holding space 20, so that thevolume of the quantity of gas enclosed in the liquid holding space, ashown in segment 40 g, is reduced rapidly and its pressure increasesrapidly accordingly.

[0075] At point 40 h, the pipette tip 10 has already been lifted up fromthe liquid (FIG. 1e). Shortly before this, the inflow of liquid into theliquid holding space 20 of the pipette tip 10 has ended (FIG. 1d′). Atpoint 40 h the quantity of gas enclosed between the plunger 16 and theliquid in the liquid holding space 20 is under a negative pressuredifference p which is approximately proportional to the quantity ofliquid dosed for sufficiently large dosed quantities of liquid. At verysmall quantities of liquid, i.e., at quantities less than approximately30 μL, depending on the liquid, friction and adhesion effects betweenthe liquid and the wall of the pipette tip are so strong that there isno direct proportionality between the negative pressure difference andthe quantity of liquid dosed.

[0076] The individual error ranges 44, 46, 48, 50, 52, 54, 56 and 58 aredelimited with respect to one another in time and by pressure valuesand/or pressure value curves over time. The pressure setpoint range 40is delimited toward lower pressures by the lower threshold curve 60 andtoward higher pressures by the upper threshold curve 62. The lower andupper threshold curves 60, 62 are functions of the pressure as afunction of time and can be defined individually. For example, thefollowing errors can be assigned to the different error ranges:

[0077] Error range 44: defective pressure measurement

[0078] Error range 46: pipette opening clogged

[0079] Error range 48: aspiration time too long

[0080] Error range 50: defective pressure measurement

[0081] Error range 52: aspiration and dispensation switched and pipetteopening clogged

[0082] Error range 54: pipette tip leaky

[0083] Error range 56: aspiration process interrupted or air bubbles inthe liquid

[0084] Error range 58: too little or no liquid in the pipette tip

[0085] The curves of the gas pressure, the pressure setpoint range anderror ranges surrounding the pressure setpoint range in the dispensingprocess are shown in time range B in FIG. 3. The dispensing process maytake place, for example, following the aspiration process describedpreviously or following a transport process in between (time range C).The same elements as in the time segment A of the aspiration process areprovided with the same reference numbers with added primes in timesegment B of the dispensing process. The error ranges in time segment Bare numbered so that the ranges with a corresponding error assignmentare designated with the same reference number plus a prime. The pressureat the time of the last liquid droplet is labeled as 40′i and theequilibrium pressure which is established after the plunger comes to astandstill and which is derived from the ambient pressure P₀ is labeledas 40′k.

[0086] The following assignment of error messages and error ranges isapplicable:

[0087] Error range 46′: pipette opening clogged

[0088] Error range 48′: dispensing time too long

[0089] Error range 52′: aspiration and dispensation mixed up

[0090] Error range 56′: pipette tip or pipetting system leaky

[0091] Use of the error ranges is to be understood as follows. Forexample, if the pipetting opening is clogged in dispensation, the liquidpresent in the pipette tip cannot come out of the pipette tip or can doso only to a limited extent. Because of outward displacement movement ofthe plunger in dispensation, which reduces the volume of the liquidholding space of the pipette tip, the gas volume enclosed in the pipettetip is compressed. Therefore, the gas pressure increases. As a resultthe curve of the pressure over time leaves the setpoint range 42′ andexceeds its upper threshold value 62′, entering the error range 46′.This is indicated by the dotted line 41′ in the time segment B in FIG.3. In this way it is not only possible to reliably ascertain that anerror has occurred during the liquid dosing process but also the errorcan be diagnosed.

[0092] In the time range in between, a pressure monitoring may also takeplace with an allowed pressure setpoint range 42″ which is somewhatenlarged toward the top and bottom to take into account allowed pressurefluctuations in transport, in particular with a jerky movement.

[0093] If the pressure exceeds the setpoint range (error range 70) or ifit falls below the setpoint range (error range 72), an error isdetected.

[0094]FIG. 4 shows the course of an evaluation of a liquid dosingprocess in a flow chart. At step S1 the pipetting process begins, e.g.,the aspiration process known from the time segment A in FIG. 3. At thebeginning of the liquid dosing process, parameters that are relevant forthe sequence are initialized, i.e., a clock is set at zero and started,the pressure P_(erf) detected by a pressure detecting sensor at adetection point in time t_(erf) is set at zero and likewise thedetection point in time t_(erf) [is set at zero]. In addition a flagF_KI which indicates in the error case whether the pressure has left thepressure setpoint range toward higher or lower pressure values is set atzero. The value clock_(max) which indicates the duration of the liquiddosing process is loaded.

[0095] In the next step S2 the pressure of the gas present in the liquidholding space is detected and the momentary value of the clock is loadedinto the variable t_(erf) of the detection point in time. The pressure pmeasured at the detection point in time t_(erf) is loaded into thevariable P_(erf), i.e., the pressure determination is performed at thepoint in time t_(erf).

[0096] In the following step S3 the threshold values assigned to therespective detection point in time t_(erf) are loaded from a memory. USWhere denotes the lower threshold value of the pressure setpoint range(i.e., the value of the lower threshold curves 60 at the point in timeterf in FIG. 3) and OSW denotes the upper threshold value. SW, throughSW_(n) denote the threshold values which separate the individual errorranges. For example if the pressure determination is performed at thepoint in time t_(erf) indicated by the line 64 in FIG. 3, then the pointSW₁ is the threshold value separating the error range 56 from the errorrange 54 and the point SW₂ is the threshold value separating the errorrange 54 from the error range 52. The value n indicates the maximumnumber of threshold values between two error ranges. In the exampleshown in FIG. 3, n=2.

[0097] In the next step S4 a check is performed to determine whether thepressure P_(erf) detected is equal to or greater than the lowerthreshold value USW which delimits the pressure setpoint range towardlower pressure values. If this is the case, then a check is performed inthe next step S5 to determine whether the detected pressure P_(erf) issmaller than or equal to the upper threshold value OSW which delimitsthe pressure setpoint range toward higher pressure values. If this isalso the case, then in the next step S6 the information that the processis taking place correctly is output.

[0098] Step S7 representing a waiting loop which allows another pressuredetermination only when the period of time t has elapsed since the lastpressure determination.

[0099] In step S8 a check is performed to determine whether or not thetime limit clock_(max) has been reached for the dosing process. If thetime limit has been reached, the sequence ends, and if not, the sequencereturns to step S2 and thus to a renewed determination of the gaspressure in the liquid holding space of the pipette tip.

[0100] If it is found in step S4 that the pressure P_(erf) detected islower than the lower threshold value USW, i.e., the curve of the gaspressure over time leaves the pressure setpoint range toward lowerpressure values, then in a step S9 the flag F_KI is set at the value 1.In the following step S10 the running variable k=1 is set. If the curveof the pressure value over time leaves the pressure setpoint rangetoward higher pressure values, i.e., if it is found in step S5 that thepressure P_(erf) detected is greater than the upper threshold value OSW,then step S10 is likewise reached but the flag F_KI remains at itsinitialization value of zero.

[0101] After it has already been determined that an error has occurredin the liquid dosing process, it is diagnosed in the steps describedbelow. The following convention is used: at least one error message isassigned to each error range. The error messages are defined as aone-dimensional field (=vector), whereby the individual entries [for an]error message (x) in the error message field are assigned to the errorranges in the direction of increasing pressure, i.e., error message (0)is assigned to error range 46, error message (1) is assigned to errorrange 56, error message (2) is assigned to error range 54 and errormessage (3) is assigned to error range 52. Accordingly, theone-dimensional error message field contains a different number ofentries as a function of the number of error ranges present at a certainpoint in time.

[0102] In step S11, the system determines whether the pressure P_(erf)detected is greater than the k^(th) threshold value. If this is thecase, in step S12 the running variable k is incremented by one and acheck is performed in step S13 to determine whether or not k has alreadyexceeded the maximum number n of threshold values assigned to the pointin time t_(erf). If k has not yet exceeded this number, then the checkof step S11 is repeated, but this time with a running variable that hasbeen incremented by one.

[0103] However, if k exceeds the value n after being incremented by one,the pressure value tested must be in the error range having the highestpressure value range and in step S14, the error message (k) is output,i.e., in the present example this is error message (3) of error range52.

[0104] If the check in step 11 reveals that the pressure P_(erf)detected does not exceed the threshold SW_(k), then in step S15 a checkis performed is performed to determine whether the flag F_KI has a valueof 1, i.e., whether the curve of the pressure overtime has broken out ofthe pressure setpoint range toward either higher or lower pressurevalues. If the flag F_KI has a value of zero, i.e., the pressure hasleft the pressure setpoint range toward higher pressure values, then theerror message (k) is output. If the check in step S15, however, revealsthat the value of the flag F_KI has a value of 1, i.e., the curve ofpressure over time has left the pressure setpoint range in the directionof lower pressure values, then in step S16 the error message (k−1) isoutput. After output of the error message, the sequence in this examplejumps to the waiting loop of step S7. However, it is also possible forthe output of an error message to be followed by another procedure,e.g., an emergency off of a pipetting system or replacement of a pipettetip. However, it is also frequently interesting in the event of a faultydosing process to monitor the course of the state variable detected overtime until the end of the dosing process, because the timecharacteristic of the at least one state variable can under somecircumstances reach several error ranges.

[0105] The device in which the inventive process takes place may be, forexample, an electronic data processing system, in particular a personalcomputer or process-controlling microcontrollers. This data processingsystem is connected to at least one sensor at the tip of the pipette todetect the time characteristic of at least one state variable, e.g., thepressure. The data memory may be a hard drive, a CD-ROM, an internal RAMmemory inside the computer system or a memory in a PC connected to themicrocontroller. For example, the state variable setpoint range may bestored on a CD-ROM. The CPU of the data processing system forms the dataprocessing unit, and a display screen or a printer constituted theoutput unit of the device. The CPU may also form an editing unit, inwhich case the data processing system will then include a keyboard or amouse or the like as an input unit for editing of the state variablesetpoint range.

[0106] The state variable measurement accompanying the dosing processmay be analyzed, e.g., according to the flow chart described above,wherein a numerical departure of the instantaneous measured value fromthe tolerance range is detected. A graphic analysis (e.g., patternrecognition technology) is also conceivable for ascertaining whether,and if so, where the momentary measurement curve departs from thetolerance band.

[0107]FIG. 5 shows the creation of state variable setpoint ranges forthe preferred embodiments of this invention. FIG. 5a shows apressure-time diagram (the time is plotted on the abscissa, pressure onthe ordinate) in which a statistically significant set of curves 70 ofpressure-time curves has been plotted; these curves were measured on adosing process which was carried out with identical equipment and withidentical working parameter settings. FIG. 5b shows a state variablesetpoint range, i.e., pressure setpoint range 142 which is limited inthe direction of higher and lower pressure values by the envelope curveof the set of curves 70 of FIG. 5a, for example. Accordingly, FIG. 5cshows a reference curve 242 which is obtained from the set of curves 70,e.g., by forming the average.

[0108] This reference curve 242 can be compared with a pressure-timecurve measured currently, namely measured in a dosing process to beevaluated, by means of correlation calculation methods such as spectralanalysis methods, preferably a fast Fourier transform and/or waveletsmethod and/or numeric convolution. The quality of the respective dosingprocess can be evaluated as a function of the degree of correspondenceresulting from this correlation (Numerical Mathematics, H. R. Schwarz,Teubner Verlag Stuttgart; “Engineering Analysis” 1 and 2, ChristianBlatter, Springer Verlag, 1996).

[0109] The degree of correspondence is usually a number which isstandardized to have a value between 0 and 1, where 1 is the value foridentical correspondence. A correspondence setpoint range extending from0.9 to 1, for example, indicates the value range for whose degree ofcorrespondence values a dosing process is evaluated as fault-free. In avalue range from 0.4 to 0.9, for example, a questionable quality of thepipetting can be assumed, in which case a decision is to be made in theindividual case as to whether or not to discard the pipetting. In theremaining value range (0 to 0.4 in this example), a serious mistake inpipetting is ascertained.

1. Method of evaluating a liquid dosing process in a container which isfilled at least partially with a gas, preferably air, in particular anaspiration and/or dispensing process in pipetting, in which process atime characteristic of at least one state variable (p) of a mediumpresent in the container is determined essentially over the entireduration of the dosing process; in which in addition essentially theentire time characteristic (40; 40′; 40″) of the at least one statevariable (p) is compared with a predetermined state variable setpointrange (42; 42′; 242) and in which an evaluation result (S6, S14, S16) isoutput dependent on the result of the comparison.
 2. Method according toclaim 1, characterized in that the medium is the gas present in thecontainer.
 3. Method according to claim 1 or 2, characterized in thatthe state variable is the pressure (p) and/or the temperature of themedium.
 4. Method according to one of the preceding claims,characterized in that the state variable setpoint range (42; 42′; 242)is defined at least for the entire duration of the liquid dosingprocess, preferably also for the duration of a transport process inbetween.
 5. Method according to one of the preceding claims,characterized in that the state variable setpoint range (42; 42′; 242)of a liquid dosing process is based on a plurality of performances (70)of essentially the same liquid dosing process using essentially the sameprocess parameters.
 6. Method according to one of claims 1 through 5,characterized in that the predetermined state variable setpoint range(42; 42′) follows a setpoint curve, and a determination is performed asto whether the time characteristic (40; 40′) of the at least one statevariable (p) is within the predetermined state variable setpoint range(42; 42′) which follows a setpoint curve and an evaluation result (S6,S14, S16) is output dependent on the result of the determination. 7.Method according to claim 6, characterized in that the liquid dosingprocess is evaluated as faulty when it is found that the timecharacteristic (40; 40′) detected for at least one state variable (p) isoutside the state variable setpoint range (42; 42′) for at least asection segments.
 8. Method according to claim 6 or 7, characterized inthat when the time characteristic (40; 40′) of the at least one statevariable (p) is outside the state variable setpoint range (42; 42′) fora section, the method determines whether in at least some segments thecurve of the at least one state variable (p) lies in at least one errorrange of a plurality of error ranges (44, 46, 48, 50, 52, 54, 56, 58,46′, 48′, 52′, 56′) of a state variable value range which is outside thestate variable setpoint range (42; 42′), and an error message is outputdependent on at last one error range (46′) which has been passedthrough.
 9. Method according to one of claims 6 through 8, characterizedin that the determination as to whether the time characteristic of theat least one state variable (p) is within the predetermined statevariable setpoint range (42; 42′) is performed by comparing thecharacteristic (40; 40′) with an upper threshold curve (62; 62′) whichdelimits the state variable setpoint range (42; 42′) toward larger statevariable values and by comparison with a lower threshold curve (60; 60′)which delimits the state variable setpoint range in the direction ofsmaller state variable values.
 10. Method according to claim 6 or 7,characterized in that the determination as to whether the timecharacteristic (40; 40′) of the at least one state variable (p) iswithin the predetermined state variable setpoint range (42; 42′) isperformed by image processing.
 11. Method according to one of claims 1through 5, characterized in that by correlation calculating methods adegree of correspondence of the time characteristic of the at least onestate variable (p) to/with a predetermined reference curve (242) as thestate variable setpoint range (142) is determined and in that anevaluation result is output as a function of the results of thedetermination.
 12. Method according to claim 11, characterized in thatthe degree of correspondence as the result of the determination is anumerical value, whereby the liquid dosing process is evaluated asfaulty when the degree of correspondence lies outside a predetermineddegree of correspondence setpoint range.
 13. Method according to one ofclaims 11 and 12, characterized in that when the degree ofcorrespondence is determined as being outside the predetermined degreeof correspondence setpoint range, a determination is performed as towhether the degree of correspondence is in an error range of a pluralityof error ranges of a degree of correspondence value range which isoutside the correspondence setpoint range, and an error message isoutput as a function of the error range in which the degree ofcorrespondence is situated.
 14. Method according to one of claims 11through 13, characterized in that the correlation calculation methoduses as input quantities interpolation points from the timecharacteristic of the at least one state variable (p) and from thereference curve.
 15. Device for evaluating a liquid dosing process in acontainer which is filled at least partially with gas, preferably air,using the method according to one of the preceding claims, whereby thedevice comprises; at least one sensor for detecting the timecharacteristic (40; 40′) of at least one state variable (p), a datamemory for storage of a predetermined state variable setpoint range (42;42′; 242) and for storing state variable values detected by the sensor(p), a data processing unit for comparing the time characteristic(40;40′) of the at least one state variable (p) with the predeterminedstate variable setpoint range (42; 42′), an output unit for output of anevaluation result (S6, S14, S16) as a function of the result of thecomparison by the data processing unit.
 16. Device according to claim15, characterized in that a plurality of predetermined error ranges (44,46, 48, 50, 52, 54, 56, 58; 46′, 48′, 52′, 56′) is stored in the datamemory, at least one possible error of the dosing process being assignedto each error range (44, 46, 48, 50, 52, 54, 56, 58; 46′, 48′, 52′,56′).
 17. Device according to one of claims 15 and 16, characterized inthat the device also includes an editing unit for creating the statevariable setpoint range.
 18. Device according to claim 17, characterizedin that the device includes an input unit connected to the editing unit.19. Device according to claim 18, characterized in that the output unitis a graphic output unit, and a state variable setpoint range isgraphically definable via the input unit.
 20. Device according to one ofclaims 15 through 19, characterized in that the data processing unitdetermines whether the time characteristic (40; 40′) of the at least onestate variable (p) is within the predetermined state variable setpointrange (42; 42′).
 21. Device according to one of claims 15 through 19,characterized in that the data processing unit performs a correlationcalculation process for determination of a degree of correspondence ofthe time characteristic of the at least one state variable with apredetermined reference curve (242) as the state variable setpoint range(142).
 22. Device according to claim 21, characterized in that apredetermined degree of correspondence setpoint range is stored in thedata memory.
 23. Device according to claims 21 and 22, characterized inthat the data processing unit determines whether the degree ofcorrespondence is within the predetermined degree of correspondencesetpoint range.
 24. Pipetting system having an evaluation deviceaccording to one of claims 15 through
 23. 25. Pipetting system in whicha pipetting process is evaluated by a method according to one or more ofclaims 1 through 14.